1. Field of the Invention
The present invention relates to a write-once information recording medium capable of recording and reproducing information by using a short wavelength laser light such as, for example, a blue laser light, and a coloring matter material for the medium.
2. Description of the Related Art
As is well known, in recent years, a medium for storing digital data has been increasingly important with the prevalence of personal computers and the like. For example, at present, an information recording medium capable of digitally recording and reproducing video information and voice information for a long period has become prevalent. Also, an information recording medium for digital recording and reproduction has been used for mobile devices such as cellular phones.
Here, as an information recording medium of this type, a disk shaped one is frequently utilized for a variety of reasons: it has a large information recording capacity; it has a high random access performance capable of making a search for desired recorded information speedily; and moreover, it is small and light in weight, has excellent portability, and is inexpensive.
As such a disk shaped information recording medium, currently, there is mainly used: a so-called optical disk capable of recording and reproducing information in a non-contact manner by emitting a laser light. This optical disk primarily conforms to a compact disk (CD) standard or a digital versatile disk (DVD) standard, and has compatibility between both of these standards.
There are three types of optical disks: a reproduction only type which cannot record information such as CD-DA (digital audio), CD-ROM (read only memory), DVD-V (video), or DVD-ROM; a write-once which can write information only once such as CD-R (recordable) or DVD-R; and a rewritable type which can rewrite information many times such as CD-RW (rewritable) or DVD-RW.
Among them, as one capable of recording information, a write-once optical disk using an organic coloring matter for a recording layer is the most prevalent because of its low manufacturing cost. This is because, if an information recording capacity exceeds 700 megabytes (MB), there is almost no use of erasing recorded information and rewriting a new item of information, and eventually recording information only once will suffice.
In a write-once optical disk using an organic coloring matter for a recording layer, after a laser light has been emitted to a recording region (track) defined by a groove, when a resin substrate is heated at its glass transition point Tg or more, the organic coloring matter film in the groove undergoes photochemical reaction, and a negative pressure is generated. As a result, a recording mark is formed by utilizing the fact that the resin substrate is deformed in the groove.
A typical organic coloring matter used for CD-R in which a wavelength of a laser light for recording and reproduction is about 780 nm includes a phthalo cyanine based coloring matter such as IRGAPHOR Ultragreen MX available from Ciba Speciality Chemicals. In addition, a typical organic coloring matter used for DVD-R in which a wavelength of a laser light for recording and reproduction is about 650 nm includes an azo metal complex based coloring matter available from Mitsubishi Chemicals Medium Co., Ltd.
In the meantime, in a next generation optical disk which achieves higher density and higher performance recording and reproduction as compared with a current optical disk, a blue laser light having a wavelength of about 450 nm is used as a laser light for recording and reproduction. However, an organic coloring matter material capable of achieving practically sufficient recording and reproducing features by using such a light beam with a small wavelength has not been developed yet.
That is, in the current optical disk for carrying out recording and reproduction by using a infrared laser light or a red laser light, there is used: an organic coloring matter material having a large absorption extremity at a wavelength side which is shorter than a wavelength (780 or 650 nm) of laser light for recording and reproduction. In this manner, the current optical disk achieves a so-called H-to-L (high-to-low) feature wherein the light reflectivity of a recording mark portion formed by emitting a laser light is lower than that before emitting the laser light.
In contrast, in the case where recording and reproduction are carried out by using a blue laser light, there is a problem that: an organic coloring matter material having an absorption extremity on a wavelength side which is shorter than a wavelength (405 nm) of the laser light for recording and reproduction is poor in stability and preservation durability relevant to an ultraviolet ray or the like; is poor in stability relevant to a heat; and is low in contrast and resolution of the recording mark.
In addition, blurring of the recording mark is probe to increase, and thus, this blurring affects the adjacent tracks, and deterioration of the cross-write feature is likely to occur. Further, there occurs inconvenience that a recording sensitivity is lowered and that a sufficient reproduction signal S/N (Signal to Noise) ratio and a bit error rate cannot be obtained.
Under a condition in which no information is recorded in the adjacent tracks, there is a case in which a predetermined recording sensitivity can be obtained. However, if information is recorded in the adjacent tracks, a reproduction signal S/N ratio is lowered because a cross write into the adjacent track is large. In addition, a bit error rate is higher, and a level suitable to practical use is not reached.
In Jpn. Pat. Appln. KOKAI Publication No. 2002-74740, there is disclosed an optical recording medium in which an absorption extremity of an organic coloring matter compound included in a recording layer exists at a wavelength side which is longer than a wavelength of a write light beam. However, in this document, there is nowhere described a configuration of improving performance of an optical disk itself such as a change of a light reflectivity before and after emission of a laser light, a reproduction signal S/N ratio, or a bit error rate.